1. Field of the Invention
The present invention relates to a systematic speech synthesizing system. More particularly, the present invention is directed to a digital speech synthesizing system which synthesizes speech which is stable and very natural and a speech synthesizing system which performs parameter interpolation during the synthesis of speech by a simply constructed critical damping two-order filter to smooth the parameter connection and thus produce natural sounding synthesized speech.
The speech synthesizing apparatus of the present invention may be used, for example, on an output apparatus for outputting things such as speech keyboard input sentences to confirm the keyboard input, typing machines for the blind, and voice answering machines using telephones.
2. Description of the Related Art
In speech Synthesis, the output sound should be as close as possible to the human voice, i.e., speech that is as natural as possible. One type of speech synthesis is systematic speech synthesis. In such speech synthesis, speech is synthesized using pulses for vowels and random numbers for consonants. In human speech, however, the voice is modulated, i.e., the voice fluctuates. For example, when stretching the vowel "ah" to "ahhh", the amplitude of the speech waveform, the pitch, frequency, etc., do not remain completely constant, but are modulated (or fluctuated). Even when changing to another sound, the apparatus, pitch, etc. do not undergo a smooth change, but are modulated. For this reason, when synthesizing speed, if the amplitude, pitch, and other parameters are kept constant at steady portions of speech and the apparatus, pitch, and other parameters smoothly change at the nonsteady portions, only a mechanical, monotonous speech can be obtained. Therefore, in the prior art, attempts have been made to modulate the output of speech synthesizers to produce very natural sounding synthesized speech.
On the other hand, when synthesizing speech, conversion occurs by inputting sentences.fwdarw.converting to sound codes.fwdarw.preparing synthesis parameters.fwdarw.outputting speech. When synthesizing speech for an arbitrary sentence, the parameters are linked in accordance with predetermined rules, waiting with each synthesis unit smaller than a single sentence, for example, speech elements or syllables, so as to form a time series of parameters. If a suitable linkage is not performed, noise occurs in the synthesized speech and the natural characteristic of the synthesized speech is lost. Therefore, the parameters of the individual speech synthesis units must be smoothly changed as in actual speech. Thus, a method for an interpolation of parameters is proposed.
All of the prior art, however, suffer from the problem that stable, very natural, modulated speech synthesis cannot be achieved. This prior art will later be explained in further detail with reference to the drawings. Further, the construction of the filters used for speech synthesis requires simplication.